A new report has suggested that U.S. women have increasingly used marijuana throughout pregnancy, often to combat the struggle of morning sickness. The trend raises concern due to evidence linking the plant's use with low birth weights and other problems. In 2014, almost 4 percent of pregnant women said they'd recently used marijuana, up from 2.4 percent in 2002, according to an analysis of annual drug use surveys. Dr. Nora Volkow, director of the National Institute on Drug Abuse, said the results raise concerns and urged doctors and health care providers to avoid recommending the plant for pregnant women.